I think you should just go in with the goal of listening to what they have to say. You don't have to commit to anything. If pressed, say that you
so appreciate that they want the best for your son and that you'd like to take some time to think about things.
If you haven't yet, I'd look into different schooling options from curriculum compacting to subject acceleration to full-grade acceleration. Are any of those things that you'd like for this year, or for next year? Do you want to make any changes for this year at all or do you only want to look at next year's options?
And remember, you're at least as much an expert on your son as they are, almost certainly moreso. You seem like you have figured out a way for your son to be academically challenged with a pretty-great-sounding every-other-day K. There is no hurry, so you can just go in open-minded to listen.
Do you have any testing or online class data or anecdotal evidence of your son's abilities or academic level that you would like to share with them?
Most of all, yay to your school for recognizing your DS's abilities and being proactive! That's a big deal!
